This invention relates to an electronically scanned phased array radar and more particularly to an apparatus and method for improving angular measurement of an antenna beam by decorrelating peak phase quantization errors of digital phase shifters in the antenna using digital randomization.
A phased array antenna comprises a plurality of radiating elements typically arranged in planar and doubly periodic grid. Such an antenna in a radar system is well adapted to electronic scanning techniques which permit a pencil beam of electromagnetic energy to be moved rapidly from one direction to another by means of a plurality of phase shifter elements.
The phased array antenna can be corporate-fed or optically-fed from one or more radio-frequency (RF) sources. Uncollimated and unsteered power from such one or more RF sources equally distributed to individual elements passes through the phase shifter device and is radiated therefrom with a phase relationship determined by the setting of the individual phase shifter so as to provide the desired collimated and steered radiated wavefront. By the reciprocity theorem the device is reciprocal, i.e., energy reflected from distant objects and impinging on the array in the form of plane wavefront will be focused by the array in a direction corresponding to the setting of the individual phase shifter.
In U.S. Pat. No. 4,445,119, entitled "Distributed Beam Steering Computers," issued Apr. 24, 1984, to George A. Works, and assigned to the present assignee, a microcomputer is co-located with each phase shifter of a phased array antenna for calculating a phase shift steering command for each element of the phased array antenna. Such a distributed microcomputer or controller approach significantly reduces wiring, cables and differential drive cards and improves reliability.
Furthermore, in the prior art, it is well known that a digital phase shifter produces a phase quantization error which increases the pointing error of the antenna beam and antenna pattern sidelobe levels. For example, in an article entitled "Minimizing the Effects of Phase Quantization Error in an Electronically Scanned Array", by C. J. Miller, Proc. of Symposium on Electronically Scanned Array Techniques and Applications, RADC-TDR-64-225, Vol. 1, Jul. 1964, pp. 17-38, Miller suggests introducing variable lengths in the lines of a corporate-fed phased array antenna in order to minimize the peak phase quantization errors. To accomplish this phase error reduction, a piece of cable or waveguide segment has been inserted in series with each phase shifter in order to decorrelate this phase quantization error. Such an approach is referred to as "cable randomization" and it has been used in phased array radar systems such as the Cobra Dane (AN/FPS-108) Radar System used by the U.S. Air Force. (See "Cobra Dane Wideband Pulse Compression System," by E. Filer and J. Hartt, Paper No. 61, 1976 IEEE EASCON, Washington, D.C., Sept. 1976, pages 26-29). In this phased array radar system, 6-bit cable randomization was implemented with a 4-bit phase shifter for peak pointing error reduction of the antenna beam at reasonable cost.
However, more recent applications of phased array radars require higher angular measurement for antenna beam steering accuracies, which require phase quantization errors of digital phase shifters to be reduced significantly. For an angle accuracy specification of 50 microradians, an 8-bit cable randomization would be required in certain applications, but 6-bit cable randomization is a practical limit.